How to Measure and Size a Dog Door for Your Back Door

Size a back-door dog door by measuring your dog’s widest body point and its shoulder height, then adding 1–2 inches of clearance to both so the flap clears the back comfortably.

Measure your dog twice and you only cut the door once. Guessing by weight, or eyeballing the opening against a wagging dog in the hallway, is how back doors end up with a hole too small for the dog that actually lives there. The whole job comes down to four numbers and one cardboard test, and none of them take more than a couple of minutes.

Here’s the good news: the measuring is the hard part, and it’s easy. Cutting the door is the part you can’t undo.

What Two Measurements Decide The Flap Size?

Two numbers decide the flap: your dog’s height at the withers (the top of the shoulders, not the head) and its width at the widest point, which is usually the chest or the hips. Add 1–2 inches to each and you have your minimum opening.

Your dog needs to stand naturally on all four paws, not sitting, not stretched out, not mid-treat. Measure from the floor straight up to the withers — the bony ridge between the shoulder blades. Keep the tape off the head entirely; a dog that lifts its head to watch you will throw the number off by inches.

For width, run the tape around the thickest part of the chest or the widest point of the hips, whichever is bigger. Most dogs measure wider at the chest than owners expect.

That clearance matters more than most people think. Hale Pet Door’s sizing guide says the flap should sit at least 1–2 inches above the pet’s back, and PlexiDor’s door finder says the finished opening height should be about 2 inches taller than the dog’s back. The gap isn’t cosmetic — a flap that grazes the spine gets dragged every time the dog pushes through, and it wears out fast.

How Do You Check The Step-Over Height?

The step-over is the rise from the floor to the bottom of the opening, and it should never be more than one-third of your dog’s height. A tall opening with a high threshold strands a short-legged dog just as surely as a small flap.

Measure your dog from the withers to the floor again, then divide by three. That number is the tallest rise the dog can clear comfortably.

The threshold really does change the math. Those rises are part of your dog’s clearance problem, not a separate detail.

Want the tested options side by side instead of a tape measure? Our roundup of back doors with dog doors built in compares the rise and flap sizes across models.

How Big Should The Door Be If You Have More Than One Dog?

Size for the largest pet that will use the door. One opening sized for a 60-pound dog comfortably serves a 20-pound one — a small dog simply has extra clearance. The reverse never works.

Puppies complicate it, and there’s no clever fix. Puppies grow past their door’s flap size on a predictable schedule, so an opening sized for the adult dog is either a second installation later or one now with room to spare. Fur counts, too, in the heavy-coat breeds; a 2-inch clearance on a coated dog can feel like a squeeze.

What Should You Check Before Cutting The Back Door?

Cut a cardboard mock-up first. Trace the flap size onto cardboard, hold it against the door at the intended height, and send the dog through it. This one step catches almost every bad measurement before the saw comes out.

Then check the door itself. Fit depends on the door material and thickness, and products specify thickness and rise options that have to match your cutout and finished opening height. Glass-door and wall installations follow different sizing and installation rules than a standard back door, so confirm which you’re working with first.

• Measure at the withers, never the head. A raised head adds phantom inches.
• Skip weight-based sizing. A dense 40-pound dog and a lanky 40-pound dog are not the same shape.
• Use the manufacturer’s chart once you have both numbers, then compare them to the flap dimensions.

PetSafe’s door measurement chart runs the same three-step method: measure the widest point and add clearance, measure the shoulder-to-chest height and add clearance, then confirm the step-over against the withers-to-floor measurement before picking a size. The PetSafe door measurement chart prints the method and the size-selection chart on one page.

One last note: every suggested measurement is a guide, not a guarantee. Final fit comes down to your dog’s comfort and the specific product’s instructions. Trust the dog moving through the cardboard cutout over any chart.